Above: Look out Toro, Horner's coming and he's packing an AK! Photo by Daniel Hendricks, licensed.
Well, I say "our," but I've never really given much of a thought to Torosaurus, beyond my 10-year-old self trying to get the mechanized DinoRiders toy. But some people seem to feel inexplicably attached to this name, often and incorrectly translated as "bull lizard" (it's got big horns, and was ornery like a raging bull!). In reality it means "perforated lizard", in reference to the holes in its frill. And it's not like the animal will cease to exist, it's just the mature, let's say "silver back" stage of Triceratops. Or should we say, Torosaurus is the mature morph, and Triceratops is the immature morph, of Agathaumas. As I mentioned before, Agathaumas specimens lack diagnostic characters in the context of an environment with multiple taxa from which it must be distinguished. But if there's only one ceratopsian in town, the identity of those generic-looking postcrania is narrowed down to one option, and it's no longer a nomen dubium.
Above: Dear E.D., Please stop naming isolated vertebrae and teeth. kthx. Photo by F. Gutekunst pre-1897, public domain.
Ah, the nomen dubium, or "doubtful name." This is an unofficial* designation given by scientists to names whose type specimens can't be classified because they're too generic. Like many dinosaurs discovered by E.D. Cope, Agathaumas was named based on partial postcranial remains. As more fossils turned up, it turned out that its "unique" postcranial features were actually characteristic of a larger clade, and all ceratopsids have nearly identical torsos and limbs. So Agathaumas lost it's standing as a valid name, because (lacking a skull) it couldn't be determined which species of ceratopsid the bones came from. True, the genus name could have simply been used for all ceratopsid species with this type of postcrania, but modern taxonomists don't roll that way, preferring genera to be mostly monophyletic. For years, Agathaumas remained known as a nomen dubium. It probably belonged to either Triceratops or Torosaurus, the only contemporary contenders known from better remains, but since no skull was found with the type specimen, we could never know for sure which one was the junior synonym.
* The ICZN does not officially recognize any such thing as a nomen dubium, and contrary to popular belief, has no rules to the effect that family names can't be based on dubious taxa, etc. Hence Titanosauridae (=Saltasauridae), Hadrosauridae (=Lambeosauridae), Troodontidae (=Stenonychosauridae), Deinodontidae (=Tyrannosauridae), Podokesauridae (=Coelophysidae) Ceratopsidae (=Chasmosauridae) and yes, Ornithodesmidae (=Dromaeosauridae) are all perfectly valid.
That is, of course, unless the number of potential synonyms is reduced to 1. With Triceratops and Torosaurus recognized as one genus, there is no reason to think that the type specimen of Agathaumas and the type specimen of Triceratops don't come from the same species. If and when another genus of ceratopsid is ever discovered from the well-sampled late Maastrichtian beds of North America, this could always be reversed, but for now the default hypothesis must be that only one was present, rather than an extremely common genus with a wide range of individual variation (Triceratops) and one shadowy mystery genus known only from a post-crania identical to Triceratops but hey, maybe it had like 20 horns or something, who knows?
Sarcasm aside, the concept of nomina dubia has become a bit crazy over the years. Jaime Headden has recently been on a mini-crusade to this point on the DML, questioning some pretty well ingrained and yet pretty flimsy concepts of what is and isn't "non-diagnostic". Let's come to our senses and realize that stratigraphic, ecological and temporal considerations also need to be taken into account. Yes, Agathaumas is non-diagnostic relative to, say, Chasmosaurus. But those two were not contemporaries, and barring the use of a time machine, the other ceratopsians that could potentially be synonyms of Agathaumas could not have existed in the same time and place as it did. Except for one, Triceratops, which by rights should go the way or either Brontosaurus (abandonment) or Coelophysis (official conservation).
Above: The dinosaur Rioarribasaurus had its name changed to Coelophysis by mistake. It turns out the original Coelophysis was not only a different species, it wasn't even a dinosaur. The original is now known as Euceolophysis, or "true Coelophysis." Photo by Ballista, licensed.
Which brings me to the promised hadrosaur implosion. Thanks largely to the work of Nicolas Campione, the taxonomy of Late Cretaceous hadrosaurs is finally being untangled after nearly two centuries of confusion. Contrary to conventional wisdom, the short-snouted type species Edmontosaurus regalis appears to be present only in the Campanian Horseshoe Canyon Formation, not the end-Mesozoic Lance, Hell Creek, etc. All the long-snouted "edmontosaurs" in the late Maastrichtian formations can be assigned to either Edmontosaurus annectens (previously Anatosaurus) or the truly duck-billed form Anatotian. Whether or not E. annectens should therefore be kept as Edmontosaurus or re-classified back to Anatosaurus is a matter of personal preference.
Above: The hadrosaur of many names, lately Anatotitan, the once and future Thespesius? Photo by Claire Houck, licensed.
But there's another monkey wrench here. Campione, in his un-published abstracts and talks so far, has been agnostic on the validity of Anatotitan as a separate species. In the past it's been suggested that the "duck-bill" is merely a preservational artifact, a crushed skull. But, there are several specimens that show this feature, and one that appears to be transitional between narrow and duck-snouted forms. On the DML, Greg Paul has asserted that there is only one species of late Maastrictian edmontosaur, and that the flat-billed versions are the mature growth stage. They even fit the pattern of growth seen in the earlier E. regalis populations, reaching the same maximum size, but then also sprouting the duckbill.
So, it looks like Anatotian is about to go the way of Torosaurus. Except, now the newly low-diversity late Maastrictian dinosaur fauna threatens to resurrect another long-dead name. Thespesius is known only from vertebrae. Those vertebrae could theoretically have come from any hadrosaur... if all hadrosaurs were immortal highlanders. In reality, though, there appears to have been only one hadrosaur species in this ecosystem. And in that case, it can have only one name. And that name is Thespesius occidentalis, "western wondrous one." That name is not Trachodon annectens, because I think the type specimen of Trachodon mirabilis is a ceratopsian. But I'm looking into that one.
Above: Some of these teeth may or may not belong to Trachodon mirabilis. From Leidy, 1860, public domain.
To continue crushing the dreams of myself in 1988 and fans of Jurassic Park everywhere, I'll next try to research whether or not Manospondylus is really a nomen oblitum like everybody is assuming (and everyone does simply assume this, because can you imagine if it weren't?). Outlook probably not so good.